This invention relates to vehicle steering systems and more particularly to a device for holding the steerable wheels of a vehicle, such as a motor home, bus, truck, automobile or the like, so that a center steering position is maintained in spite of spurious steering inputs, such as those caused by variable crosswinds, crown curvature or slant of the highway, or other factors tending to adversely affect vehicle steering by the driver.
The steering systems of highway vehicles and the like are designed primarily for driver a control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
Vehicles with steering systems having positive caster generally track relatively straight ahead and generally resist normal steering inputs away from center, including those of the driver. Intentional turning maneuvers by the driver therefore require sufficient turning force to overcome this positive resistance to movement away from center. When the driver relaxes the turning force applied to the steering wheel, a positive caster system has a definite tendency to return to its straight ahead position, although it may overshoot the neutral or center position if the steering wheel is entirely released.
While positive caster is desirable in some respects, it is not without compromises over the full steering spectrum. For example, the adverse effects of strong gusty cross winds are usually more pronounced with large amounts of positive caster. As its name would imply, the vehicle tends to caster towards the side of the roadway to which it is being pushed by the wind. Thus, the adverse steering inputs caused by crosswinds are directly related to the amount of positive caster offset, which is a classic example of having to balance a benefit with a detriment. The small amount of stability gained from castering the steerable wheels on a non-windy day may be paid for many times over when driving in a crosswind because of the destabilizing effect of the crosswind caused by positive caster offset. Positive caster offset also allows steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander, which is a universal driving complaint, particularly by driver""s of heavy vehicles such as trucks and motor homes. Similarly, a high crown at the center of the roadway or a slanted roadway can cause vehicles to turn toward the edge of the roadway, that is, in the downhill direction. In addition, generous positive caster provides significant resistance to small radius turns, which can make city driving quite fatiguing. These adverse effects are some of the negative aspects of achieving steering stability through generous amounts of positive caster.
For the lack of a more advanced method, steerable wheel castering has been accepted by the industry as a low-cost method of achieving steerable wheel returnability. Accordingly, many over-the-road vehicles are provided with generous amounts of positive caster. Not much thought has been given to the self-defeating side effects of steerable wheel castering. The failure of the industry to recognize the critical need to provide directional stability by replacing steerable wheel castering with another method of achieving steerable wheel returnability may go down in history as one of the longest enduring vehicle design oversights.
Thus, a highly important consideration that has long been overlooked by the industry is that steerable wheel castering is directly responsible for road wander, crowned road steering wheel pull and cross wind steering problems. Keeping a vehicle tracking straight and under control currently requires an inordinate amount of driver steering corrections to counteract the adverse side effects of castered wheels. The repetitive task of making thousands of precise steering corrections mile after mile weighs heavily on a driver""s physical and mental well-being, and may result in extreme driving fatigue. Thus, vehicle directional stability can best be achieved by stabilizing the on-center behavior of the steerable wheels with a more suitable method than the traditional steerable wheel castering used on many current production vehicles.
Another drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly before encountering any stabilizing resistance from the steering wheel. The transmission of these inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore produce driver fatigue.
The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver""s steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.
The invention provides a center stabilizer assembly for improved on-center holding of the steerable wheels, and significantly reduces driver fatigue because it results in a major reduction in driver steering inputs. The stabilizer assembly is easily activated by the driver while driving the vehicle, and its activation makes driving more pleasurable and less fatiguing. The stabilizer assembly comprises linkage means of variable length that extends between the steerable wheels and an axle or frame member such that the length of the linkage means defines the center position of the steering system. The linkage means comprises a resistance unit that provides a resistance force for resisting steering forces tending to move the steerable wheels to either side of the center position, and a trim unit for transmitting the steering forces to the resistance unit.
The trim unit comprises a trim piston, a trim cylinder providing first and second trim chambers, one on each side of the trim piston, a fluid transfer system for providing a flow of fluid to and from each of the trim chambers, and a solenoid operated valve for controlling the fluid flow. The control valve is operable between a closed position for preventing the fluid flow to hold the piston in a locked centering position, and an open position for allowing the piston to move to a new centering position in the trim cylinder. Movement of the trim piston causes fluid flow to one of the trim chambers and fluid flow from the other of the trim chambers. This fluid flow permits the length of the linkage assembly to change relatively freely in response to steering forces, which in turn permits the steerable wheels to move freely to a new center position in response to an applied steering force.
The resistance unit includes a component that moves with the steering system in response to steering wheel movement, and resistance to movement of this component provides a resistance force opposing relatively small movements of the steerable wheels to either side of their center position. These small movements correspond to the very large radius turns that occur when a vehicle is steered through maneuvers at highway speeds (as opposed to the small radius turns that occur when a vehicle turns a corner). Thus, during large radius turns, the resistance unit provides a resistance force that biases the steerable wheels back toward their center position, and this bias serves as a return force to return the steerable wheels to their center position upon removal of the steering force producing the large radius turn. On the other hand, during small radius turns, the resistance unit may be rendered ineffective to permit easy, away from center movements during such turns.
More specifically, the stabilizer has a pair of detent members with opposing faces, each with at least one centering detent. At least one bearing member is arranged to simultaneously contact rim bands around undercut portions of two opposing centering detents when a steerable member is in its preselected position. One of the detent members is connected to the steerable member and the other of these members is connected to the frame of the vehicle so that the bearing member and the detent members move relative to each other in response to movement of the steerable member away from its preselected position. A compressed spring is arranged to press the bearing member between the two opposing detent members so that sufficient contact pressure is maintained at all times to keep the bearing member firmly within the centering detent or in a groove defining a corresponding track in the face of each detent member for guiding the bearing member when it is moved outside of the detent. There are two tracks associated with each detent, one extending away in a direction opposite from the other. Each detent includes two sloped ramp segments each with one end adjacent to the centering detent and the other end fared into a corresponding one of the tracks, which may be flat (no slope) for providing substantially zero resistance or may have a gradually sloped (constant or changing) portion for providing a relatively small amount of resistance as described below. The ramp is also formed by a groove and the track and ramp grooves both have substantially the same radius of curvature as the bearing member so as to snugly fit and frictionally engage the bearing member to cause it to travel out of the detent, up the ramp, and along the track when the steering force exceeds a break away level of resistance.
There are preferably a plurality of bearings and a plurality of opposing of detent pairs, one pair being associated with each bearing. The pressing force between the bearing members and their centering detents resists relative movement between the bearing members and the detent members, and the pressing force provided by the compression spring is preferable supplemented by air pressure in a piston chamber adjacent one of the detent members that is in the form of a detent piston. Because the bearing and detent members are arranged between the steerable member and the frame of the vehicle, resistance to relative movement between these members prevents substantial movement of the steerable member away from its preselected center position until the steering force applied to the steering system exceeds a predetermined value corresponding to the level of break away resistance provided by the contact pressure between the bearing members and the centering detents.
The detent members are preferably opposing plates, namely a rotary plate arranged for rotary movement relative to a piston plate restrained from rotation. The bearing members are preferable a plurality of spherical ball bearings arranged in spaced relation to each other with a disc-like separator retaining this spaced relation. The bearings may have other shapes with curved outer surfaces, such as an oval shape. Each detent plate has a plurality of centering detents arranged relative to the bearing members and bearing retainer so that one of the bearings is received in each opposing pair of centering detents when the steering system is centered. The detents or depressions in each detent plate have a spaced relation corresponding to the spaced relation of the bearings. The bearings are pressed into the centering detents of the detent plates by a retaining spring to keep the plates and bearings in position and by means of air pressure in a cylindrical resistance chamber adjacent the piston plate, which serves as a reciprocating piston. The contact pressure between the bearing members and the detent plates, and thereby the resistance force, may be varied by varying the air pressure in the resistance chamber.
The invention also includes a feature for eliminating mechanical slack in the interaction between each bearing member and its corresponding centering detent. This slack-removal feature comprises providing each detent with an undercut bottom portion having a radius of curvature that is smaller than the radius of curvature of the bearing member. A narrow contact band may also be provided around the rim of the undercut portion. Although it may be slightly rounded by a convex shape, the width of this band extends generally along a line tangent to the curved surface of the bearing member, such that contact between the bearing member and the centering detent will occur substantially only along a line of contact. Where the transition between the rim of the undercut portion and the adjacent surface of the detent ramp would otherwise be relatively sharp, the contact band along which contact pressure occurs may be slightly convex (rounded) so as to minimize wear at the rim of the undercut portion. If this transition is relatively sharp, repeated travel of the bearing member over the rim may wear off the sharp edge in an uneven manner, resulting in intermittent bands of contact separated by areas of no contact. In each of these alternatives, contact between each bearing member and the rim of the undercut portion of its corresponding detent occurs along substantially a continuous line of contact.
The stabilizer includes means for remotely and selectively varying both the amount of resistance to movement away from center and the preselected position of the steerable member relative to the vehicle frame. Both of these remote adjustments can be made by the driver while the vehicle is in operation. A control system is employed for operating a solenoid and a pressure regulator and the switch and dial for actuating these devices are preferably located at the driver""s station of the vehicle. The switch preferably has a toggle that is biased by a spring to the circuit opening position. These types of switches are closed only momentarily when the toggle is held in a depressed position against the spring bias. Thus, the solenoid is actuated only while the toggle is actually depressed. Release of the toggle opens the circuit and stops the adjustment at the point selected.
The level of resistance to movement away from center may be remotely adjusted either by such a manual control system operable by the driver or by a microprocessor control system responsive to the speed of the vehicle. Thus, the turning resistance of the present invention is readily adjustable to provide a low level at low speeds and a high level at speeds of about 35 mph or greater. In this regard, the centering stabilizer of the present invention is much less complex than prior art arrangements, such as those which combine high positive caster near the center position and complex power steering systems for varying the level of power assist from a low assist level for large radius turns to a high assist level for small radius turns.
The centering return force provided by positive wheel caster follows a force curve that may provide relatively little, if any, turning resistance in the straight ahead position or for large radius turns immediately adjacent to the straight ahead position. The bearings and detent plates are sized and the centering detents are sized and shaped or xe2x80x9ccutxe2x80x9d so as to provide a resistance force which blends with any return force provided by the normal geometry of the front end of a motor vehicle. The invention can increase substantially the turning resistance available at and immediately adjacent to either side of the straight ahead position of the steerable wheels. At greater turning angles (small radius turns), the resistance force provided by the invention preferably tapers off as positive caster return force increases. The turning resistance provided by the invention at or near the centered wheel position should be sufficiently large to resist spurious steering inputs generated either by the driver or by an overactive power steering system.
In a preferred embodiment, the shape of the centering detent and other stabilizer parameters are chosen so that a total break away steering force of at least 100 pounds, preferably at least 200 pounds, and more preferably at least 300 pounds must be applied to the tie rod in order to initiate break away turning movement of the steerable wheels at vehicle speeds above about 35 miles per hour. For city driving at vehicle speeds of about 35 miles per hour or less, the break away force required is preferably lowered to about 100 pounds, more preferably below about 50 pounds, at the tie rod. Where steerable wheels are provided with positive caster, which is usually the case with highway motor vehicles, the grooves in which the bearing members ride adjacent to the upper end of the ramp of the centering detents are shaped to form a neutral (no slope) cam surface which provides no further turning resistance. In other words, interaction between a bearing member and its corresponding detent in the detent member provides a decreasing level of resistance force as the steerable wheels move away from center, until the caster return force, which increases in proportion to turning angle with positive caster, is of sufficient magnitude to alone provide stabilizing resistance. However, the resistance force need not go to zero, but instead the slope of the track groove surfaces beyond the outer ends of the ramps may provide a resistance force effective over the entire range of turning angles, which for highway vehicles is usually limited to 45 degrees on either side of the straight ahead position (the xe2x80x9c0xe2x80x9d position). Preferably, there should be sufficient positive caster for the resistance force to be effective over the range of 0-10 degrees, more preferably 0-5 degrees and most preferably 0-3 degrees on either side of center.
The stabilizer is preferably connected between the steering system and the front axle or a nearby frame member of the vehicle in a position that allows the steerable member(s) to move through its full range of steering movements while providing sufficient leverage for the apparatus to resist movement of the steerable member away from the center position producing straight ahead travel of the vehicle. The steering system connection may be made to any steering system component providing appropriate range and leverage, such as a tie rod which joins the two front steerable wheels of a highway vehicle, or the pitman arm connected to the reduction gear. The frame connection may be made to any component serving as a fixed mounting relative to the steering system.
The invention may be used with steering systems having a reduction gear between the steering wheel and the steerable wheels. In this application, the stabilizer is preferably connected to the steering system at a location between the steerable wheels and the reduction gear so as to be unaffected by any slack in the reduction gear or in components and connections between the reduction gear and the steering wheel. It is therefore on the slow side of the reduction gear ratio. The invention thus provides a zero backlash center stabilizer assembly.
The level of steering force required to initiate or breakaway into a steering movement away from center is sometimes referred to in this specification as the xe2x80x9cbreak away resistancexe2x80x9d. Different levels of break away resistance and of resistance force may be appropriate to compensate for changes in the forces acting upon the vehicle. Thus, the resistance force provided by the invention may be increased or decreased to provide a level of force sufficient to overcome any spurious steering inputs and to suit driver road feel, particularly a feel of the steering wheel that lets the driver know when the steered wheels are beginning to move away from center and are closely approaching return to center. In other words, the invention provides a distinctive feel when approaching or leaving the center position. Thus, the sense of touch is added to the visual sense to aid control of the vehicle and reduce driver fatigue.
In the absence of the invention, spurious inputs to and/or mechanical slack in the steering assembly require almost constant manipulation of the steering wheel by the driver and make it almost impossible for the driver to hold the vehicle on a true straight ahead course. Use of the invention therefore permits a substantial reduction or elimination of the caster angle of vehicles with positive caster, thereby significantly reducing the crosswind effect and providing the driver with a positive touch control not heretofore attainable with positive caster. Positive stability is thereby achieved for previously unstable steering systems.
Although the present invention is particularly useful as a center stabilizer assembly for motor vehicles, it can be employed to hold the center position of any steerable member moveable to either side of a preselected position. For example, the stabilizer can keep an outboard motor centered so that a boat follows a straight course over the water in the presence of spurious steering forces produced by wind and wave action. The stabilizer can also be used to keep centered such steerable members as the rudders of ships or airplanes and the tongues of tandem trailers or railway cars. The stabilizer is useable with both power and non-powered steering systems, with the level of resistance forces provided usually being less for vehicles without power steering.